1. Field of the Invention
The present invention relates to an artificial knee joint which is used as a replacement of a biological knee joint.
2. Prior Art
In cases where a knee joint has ceased to function as a result of deformative joint disorders, rheumatism or external injury, etc., this knee joint is replaced by an artificial knee joint.
Such an artificial knee joint is substantially comprised of a femoral component in which two protruding surfaces, i.e., medial and lateral protruding surfaces, are joined in a front and back relationship to form a femoral condylar portion, and a tibial component in which recessed surfaces that support the femoral condylar portion so that the femoral condylar portion is capable of a sliding movement and a rolling movement are joined in a front and back relationship to form a tibial condylar portion. In this case, the femoral condylar portion has an medial condylar section and an lateral condylar section, and both of these portions are formed so that the Trajectory connecting the lowest points of the two portions constitutes an approximate circular-arc curve in two dimensions as seen in the side view. In a conventional artificial knee joint, imaginary extended lines of this approximate circular-arc curve in the anteroposterior direction are set parallel to each other.
This parallel setting sets limitations on the region of possible movement of the artificial knee joint. In particular, it is difficult to achieve maximum flexion. Examples of the deleterious effects of such a construction may be cited as follows:
1. The tibial condylar portion that supports the femoral condylar portion forms an elliptical groove that has its long axis in the anteroposterior direction as seen in a plan view. The femoral condylar portion rotates while sliding and rolling through this elliptical groove, thus allowing flexion of the knee. In this case, the femoral condylar portion is contained within the vertically projected plane of the tibial condylar portion, and does not protrude from this plane in extension. However, since the center of rotation of the femoral condylar portion moves to the rear as the angle of flexion increases, the femoral condylar portion protrudes from the vertically projected plane of the elliptical surface of the tibial condylar portion. Since the edge of the femoral condylar portion, and especially the posterior portion, contacts the running part of the hamstring on both the medial and lateral, this protrusion of the femoral condylar portion interferes with the normal tension of the hamstring, and therefore impedes flexion.
2. In a biological knee joint, the tibial condylar portion is more or less planar in the anteroposterior direction, and the attitude of the femur during flexion is controlled by the anterior and posterior cruciate ligaments. More specifically, the posterior cruciate ligaments gradually extend up to a flexion angle of approximately 60xc2x0, and then remain more or less fixed, so that the attitude is controlled. In an artificial knee joint, on the other hand, the side-surface shape of the tibial condylar portion is formed with the standing position of 0xc2x0 as the deepest point, and with recessed shapes showing the shape of a xe2x80x9cship hullxe2x80x9d before and after this deepest point, so that smooth rotation of the femoral condylar portion is achieved. In this case as well, a semi-constrained type configuration in which the curvature radius of the recessed shapes of the tibial condylar portion is greater than the curvature radius of the protruding shapes of the femoral condylar portion is most common. Consequently, as the femoral condylar portion bends, the position of the contact surface with the tibial condylar portion gradually becomes higher, which differs from the movement of a biological knee joint. Accordingly, when the cruciate ligaments are retained, the extension of these ligaments becomes excessive, so that attitude control becomes difficult to achieve. Even in cases where the cruciate ligaments are excised, the extension of the medial and lateral collateral ligaments is excessive, and smooth flexion is impeded.
3. In order for the femoral condylar portion to obtain a large flexion angle, the condylar portion must be correspondingly extended upward and to posterior. However, in the case of parallel setting that requires a wide area, this interferes with ligaments and tendons that are present in the upper posterior area, so that this extension cannot be made very long. As a result, the flexion angle in a conventional artificial knee joint is limited to approximately 110 to 120xc2x0. However, flexion of approximately 150xc2x0 is necessary for, for instance, an upright sitting on a plane surface or a Japanese-style sitting position, etc.
4. In a biological knee joint, the smoothness of flexion is aided by an internal and external axial rotation movement of the femur as the flexion angle increases. However, if the femoral condylar portion and tibial condylar portion are set parallel to each other, this internal and external axial rotation movement is restricted. In other words, when the flexion angle increases, the femoral condylar portion leaves the recessed groove of the tibial condylar portion and is lifted upward, so that normal axial rotation is hindered, and the cruciate ligaments and medial and lateral collateral ligaments are excessively extended. Thus, smooth flexion and axial rotation are difficult.
The object of the present invention is to solve the above problems.
The above object is accomplished by a unique structure for an artificial knee joint in which the knee joint comprises:
a femoral component comprising a femoral condylar portion that consists of an medial condylar section and an lateral condylar section, the medial and lateral condylar sections extending in a anteroposterior direction of the knee joint and have protruding exterior surfaces; and
a tibial component comprising a tibial condylar portion that consists of an medial condylar section and lateral condylar section, the medial and lateral condylar sections extending in a anteroposterior direction of the knee joint and have recessed interior surfaces so as to support the medial and lateral condylar sections of the femoral condylar portion in such a manner that the medial and lateral condylar sections of the femoral condylar portion can make sliding and rolling movements; and wherein
a spacing between the respective medial condylar sections and lateral condylar sections of the femoral condylar portion and of the tibial condylar portion is formed so as to be gradually narrower toward the posterior side of the knee joint, thus forming a xe2x80x9crear-inxe2x80x9d configuration.
With the respective medial condylar sections and lateral condylar sections of the femoral condylar portion and tibial condylar portion formed in a xe2x80x9crear-inxe2x80x9d configuration, which is a characterizing feature of the present invention, the following advantages are obtained:
1. Since the spacing of the femoral condylar portions is formed in a xe2x80x9crear-inxe2x80x9d configuration, the spacing (distance of separation) of the femoral medial condylar section and lateral condylar section decreases as the flexion angle increases. Accordingly, even in the case of a high degree of flexion, the femoral condylar portion does not protrude from the tibial condylar portion, but remains within the vertically projected plane of the tibial condylar portion. As a result, there is no interference with the hamstring or impairment of the normal tension of the hamstring.
2. This means that the degree to which the femoral condylar portion is elevated from the tibial condylar portion is small even in the case of a high degree of flexion, so that the tension of the collateral ligaments is not extended, thus allowing smooth flexion.
3. In the xe2x80x9crear-inxe2x80x9d configuration as well, the load applied to the knee joint during flexion is applied to the positions of the centers of gravity of the medial and lateral condylar sections. Accordingly, when a large flexion angle is adopted, the center of gravity move to the medial or lateral of the top line that connects the lowest points of the condylar portions, and the curvature radius is correspondingly decreased, because the mediolateral radius of the femoral component portions are smaller than the anteroposterior. If the curvature radius is small, the amount of displacement of the center of rotation is also correspondingly decreased, so that a large flexion angle can be obtained with a small displacement, thus allowing a high degree of flexion. Furthermore, if the rotational radius is small, flexion can be accomplished by means of a light force.
4. With the xe2x80x9crear-inxe2x80x9d configuration, the axial rotation of the femur about the axis of the tibia can also be accomplished by means of a light force, since the rotational radius is reduced. Furthermore, the joint is stable with no oscillation, etc. Accordingly, the flexion action is aided, and a large flexion angle can be obtained.